Comparing the Su-35 issue to the Su-57 is really like comparing apples to oranges. There are several reasons why
1) The Su-35 at the time was a mature and completed product which was in active service in the Russian Air Force, in comparison to the Su-57 which for all intents and purpose stuck in a perpetual limbo of a development mire.
2) The Su-35 price tag and maintenance requirements are reasonable enough for countries like Vietnam and India to consider purchasing them in large numbers. So it would make sense for China to purchase some Su-35 for DACT.
On the issue on China getting the Su-35 for the engines which apparently alot of people like to harp on, I only have this to say : If the whole Su-35 purchase was solely for the engines, then Russia had cut itself one of the worse deals I have seen in the history of military sales. For a measly 2 billion USD, Russia would had given away an potentially very lucrative money tree in the form of allowing China to license produce the engines for their own fighters, the amount of Chinese fighters that would need an engine of such a caliber would be such that it will make that 2 billion dollar price tag look like mere chicken feed. Ultimately why China bought the Su-35 is one that can never be satisfactorily answered.
Back to the Su-57 topic, whether China will get the Su-57 is still an open question. Though I am leaning towards nay rather then yay, but China will have less of a reason to get the Su-57 for its engines given that the Su-57 uses a derivative of the one found on the Su-35S already. As for the issue of DACT, China already have the J-31 which can mimick the F-22 and F-35 even better than the Su-57 due to its closer external similarities. So the only role I can think off is the extremely remote possibility that another country with issues with China would buy the Su-57.
Russia would like alot of things, and China patronizing its military hardware would be one of them. But liking one thing does not make it real.
There's no real mystery regarding China's purchase of a batch of 24 Su-35's, they want capability NOW, not 4 years from now, or some point out into the future. The Su-35 offers a top end 4++ generation fighter aircraft with OVT and a good radar....
So the Su-57 offers up the same kind of "high end" fighter aircraft with OVT for super-maneuverability, some advanced avionics, designed to enhance L/O, and for good measure an internal weapons bay, with the possibility that in the future it will have a very powerful high thrust powerplant.
So that is actually more "capability now", the communist Chinese have been buying Russian military hardware and technical assistance from the get-go, its been a very successful arrangement for both parties, and with the US as a common op-for, this will further cement this long standing partnership...
so will it happen? there's really no way to know, but there are benefits for both teams if it does....